1. Field of the Invention
This invention relates to high-strength lead frame materials for a semi-conductor device which have a higher strength than conventional ones.
2. Related Art
Recently, with higher capacity and higher integration of semi-conductors (e.g. a logic device) and also with thinner design of a packages, lead frames tend not only to have an increased number of pins, but also to be formed into a thinner strip-like configuration. Therefore, lead frame materials have now been required to have a higher strength.
Fe-42Ni alloy and Fe-29Ni-17Co alloy (KOVAR.RTM.) are known as such a multi-pin Fe-based alloy lead frame material. Improved Fe-Ni alloy and Fe-Ni-Co alloy for such a lead frame material are disclosed in Japanese Laid-Open Patent Publication No. 55-131155, and high-strength Fe-Ni alloys incorporating various strengthening elements have also been proposed. Also, improved Fe-Ni-Co alloys are disclosed in Japanese Laid-Open Patent Publication Nos. 55-128565, 57-82455 and 56-6251, Japanese Patent Publication Nos. 1-817 and 1-15562 and Japanese Laid-Open Patent Application No. 1-61042. The last-mentioned Publication, that is, 1-61042, was earlier filed by the Applicant of the present application.
A multi-pin lead frame is produced mainly by a photo-etching method which enables fine working. However, in such thin strip-like multi-pin lead frame of Fe-42Ni alloy or Fe-29Ni-17Co alloy, the leads are liable to be warped or bent because of their inadequate strength during the assembling of the package and the transfer and mounting of the lead frame. Also, the leads may be buckled by an impact occurring during use.
With respect to improvements in Fe-Ni alloy and Fe-Ni-Co alloy, it has been disclosed to strengthen such alloys by the addition of Si, Mn and Cr (Japanese Laid-Open Patent Publication No. 55-131155) or by the addition of other strengthening elements. The thermal expansion of Fe-Ni-Co alloys is dealt with in (a) Japanese Laid-Open Patent Publication No. 55-128565, (b) Japanese Laid-Open Patent Publication No. 57-82455, (c) Japanese Laid-Open Patent Publication No. 56-6251, (d) Japanese Patent Publication No. 1-817, (e) Japanese Patent Publication No. 1-15562 and (f) Japanese Laid-Open Patent Publication No. 1-61042. However, in the former, since an excessive amount of the strengthening elements are contained in addition to the main elements, a surface oxidation is liable to occur, which results in a problem that the solderability and platability (which are main properties of the lead frame) are much deteriorated. With respect to the latter, none of publications except for (a) are intended to positively improve the strength of the lead frame. The technique disclosed in the above publication (a) is different in strengthening mechanism from the present invention.